Multiple-articulated robot control apparatus

ABSTRACT

A multiple-articulated robot control apparatus eliminates robot arm vibration by feedback-compensating disturbance due to interference torque when the robot arms are driven. The apparatus includes an arithmetic circuit (8) for computing mutual interference torque values for respective ones of the arms, a status observing circuit (2) for reproducing a status variable from a torque command and actual velocity of each servomotor, a conversion circuit (4) for converting an output of the status observing means into a corrective value of torque produced by disturbance acting upon the servomotor, and a correcting circuit (9) for correcting an error signal of the torque command of the servomotor by the converted corrective value and the interference torque value. Disturbance of each arm with regard to driving torque is eliminated. The apparatus is adapted to correct estimated disturbance torque.

BACKGROUND OF THE INVENTION

This invention relates to a multiple-articulated robot control apparatusfor eliminating robot arm vibration by effecting feedback compensationof disturbance due to interference torque when the robot arm is driven.

An ordinary variable program robot possesses feedback controllers whichmove the joints of a manipulator from a present value to a designatedtarget value. In accordance with these controllers, the effect whichmotion along one axis has on other axes is regarded as disturbance inthe feedback control system without producing a model of the detaileddynamics of the mutually interfering joints when deciding an actuatorsignal.

FIG. 3 is a block diagram of a feedback control system. In order tocompensate for accumulation of the error due to the abovementioneddisturbance on a realtime basis, actual motion is measured, an outputfrom an arm servomotor a serving as a plant is compared with a targetresponse and is subjected to computations by a control rule b having apredetermined feedback gain with respect to the error, and the input tothe plant is varied dynamically. However, in a case where a torquesignal for the arm required for a robot activity is applied to amultiple-articulated robot as the abovementioned actuator signal, thetorques for the respective axes generated as the actual responseinterfere with one another. The arm vibration thus brought aboutincludes frictional and gravitational forces having non-linearcomponents. In particular, in terms of eliminating path error in awelding operation or the like, it is essential that these be accuratelydetermined.

With regard to arm vibration caused when each arm is feedback-controlledby such a joint driving servomotor, the conventional practice is, say,to compute each axial torque based on a movement command along each axisand correct the torque signal applied to the arm.

The abovementioned feedback control rule requires realtime computation.Though each servomotor provides feedback with regard to a velocitysignal in such case, an acceleration signal cannot be obtained unless aposition command signal along each axis is differentiated twice.Further, in a conventional control method in which a torque value iscomputed from this acceleration and velocity, about 16 ms of time isrequired as the computation period. Accordingly, a problem encounteredis that vibration components having high frequencies cannot be removed.

SUMMARY OF THE INVENTION

The present invention has been devised in order to solve the foregoingproblems and its object is to provide a multiple-articulated robotcontrol apparatus in which an interference torque value, which causesrobot arm vibration is computed at high speed along each axis, therebymaking it possible to positively eliminate arm vibration.

According to the present invention, there can be provided a controlapparatus for a multiple-articulated robot in which each arm isindependently driven by a feedback-controlled joint driving servomotor,comprising arithmetic means for computing mutual interference torquevalues for respective ones of the arms, status observing means forreproducing a status variable from a torque command and actual velocityof each servomotor, conversion means for converting an output of thestatus observing means into a corrective value of torque produced bydisturbance acting upon the servomotor, and correcting means forcorrecting an error signal of the torque command of the servomotor bythe converted corrective value and the interference torque value,whereby disturbance of each arm with regard to driving torque iseliminated.

Accordingly, the multiple-articulated robot control apparatus of thepresent invention is such that an interference torque value is decidedfrom a torque command along each axis, a disturbance torque actingnon-linearly upon the feedback system of each axis is estimated by thestatus observing means, and these disturbance torques are employed ascompensating torques along the respective axes with regard to erroneouscommands in the velocity loop. In this way interference torque valueswhich cause robot arm vibration are subjected to feedback compensationso that arm vibration can be positively eliminated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an embodiment of a multiple articulatedrobot control apparatus according to the present invention;

FIG. 2 is a schematic view of a two-link planar manipulator havingrotating joints; and

FIG. 3 is a block diagram of a feedback control system.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will now be described in detailwith reference to the drawings.

FIG. 2 is a schematic view of a two-link planar manipulator havingrotational joints. It is possible to expand and apply the particularsdescribed hereinbelow regarding the abovementioned manipulator to amultiple-articulated robot having multiple joints. A first arm l₁ and asecond arm l₂ have two joints the positions of which can be defined byangles θ₁ and θ₂, and the axis of each joint is parallel to the Z axis.That is, the arm of the abovementioned robot is assumed to move in theX-Y plane.

These joints are arranged so as to be capable of being drivensimultaneously by the respective servomotors. The torques generatedthereby interfere with one another and cause the arm to vibrate. Adisturbance torque h(θ,θ) which acts upon the arm of the robot can ingeneral be expressed as follows based on a manipulator motion equation:

    T=M(θ)θ+Vθ+h(θ,θ)+g(θ)

Let us consider only dynamic torque caused by actualization of motion,with the term g due to gravity and the term V due to friction beingeliminated.

As the dynamic torques, an inertial matrix is indicated by M(θ), andother dynamic torques are indicated by h(θ,θ). If M(θ) is a regularmatrix, then velocity can be expressed by the following equation:

    θ=M-.sup.1 (θ)T-M.sup.-1 (θ)·h(θ,θ)

There are three types of dynamic torques considered as disturbancetorques that act so as to cause arm vibration. One is inertial torqueproportional to joint acceleration, one is centripedal torqueproportional to the square of the joint angular velocity, and one isCoriolis torque proportional to the product of joint velocities from thetwo different links.

The inertial torque arises from an ordinary action/reaction force whenthe arm is accelerated, and the centripedal torque arises from rotationconstrained about a certain center. For example, the forearm isconstrained to rotate about the shoulder joint, so that a centripedalforce is produced along the first arm l₁ in the direction of theshoulder joint. The Coriolis torque is a vortex-like force produced withinterference between the two rotational systems as the cause.

Besides interference torques caused by these dynamic torques generatedat the time of drive, disturbance torque due to friction and gravitymust be computed in order to accurately decide the parameters of theservomotors for the respective axes. Accordingly, in the presentinvention, an observer of the first axis (θ₁) is constructed as shown inFIG. 1, whereby an estimation of velocity is performed by the observerwith regard to the servomotor of the first axis, assuming that the firstaxis of the robot is driven at the same time as the second axis.

In this servo system, first an error between a commanded velocity Vcmdand the output of an observer 2 is obtained by an adder 1. The output ofan element 3, which multiplies the error by a loop gain K₁ and thenperforms integration, and the output of an element 4 for a velocity loopgain K₂, are added by an adder 5 to form a torque command U₁. The torqueU₁ obtained is input to a torque constant element 6 of a specificinertial matrix and then applied as a command signal to a servomotor 7,which is the object under control. A servomotor control processor 8provided for each axis performs the computation

    C.sub.2 ×U.sub.2 +C.sub.3 ×θ.sub.2

as a corrective torque which includes a corrective term h(θ,θ)conforming to the non-linear torque component acting on the arm, andcomponents C₂ and C₃ of the inertial matrix M(θ). This is added to thetorque command U₁ in an adder 9. Here U₂ is a torque command for theservomotor of the second axis.

An interference torque

    M.sub.1 ×θ.sub.2 +M.sub.2 ×θ.sub.2

stipulated by the velocity θ₂ and acceleration θ₂ of the arm of thesecond axis acts as disturbance upon the first axis servomotor 7, whichis under control. Here M₁ and M₂ are interference torque coefficients.

The observer 2 for this servo system is composed of elements similar tothose of the servomotor 7 and is adapted to multiply an error E withactual velocity by a predetermined coefficient and add the result to thetorque command U₁.

More specifically, in the observer 2, the torque command U₁ is convertedinto an acceleration value by a constant term 11 corresponding toinertia. This value is converted into a velocity value by an integrationelement 12. The error E is calculated by an adder 13 from the velocityvalue from the integration element 12 and the actual velocity θ₁ of theservomotor 7, and a steady error component is eliminated by elements 14and 15 having observer gains K₃ and K₄. With regard to a non-steadyerror component, addition is performed by an adder 18 via elements 16and 17 having corrective torque coefficients C₁ a and C₁ b. The resultis added to the torque command U₁ by the adder 9.

In other words, due to a non-steady error component caused by non-lineardisturbance, the error E between the output of the observer 2 and theactual velocity θ₁ of the servomotor 7 will not become zero even if allof the set parameters of the servo system are accurate and thedisturbance torque also is correctly computed along each axis.Therefore, the disturbance due to interference is removed and acorrective value is added to the disturbance torque estimated by theobserver. In this way it is possible to reduce vibration due tointerference among the arms of a multiple-articulated robot.

Though an embodiment of the present invention has been described, theinvention is not limited thereto but can be modified in a variety ofways without departing from the scope of the claims.

The control apparatus for a multiple- articulated robot according to thepresent invention is adapted to estimate disturbance torque by anobserver and add a corrective torque thereto in order to reduce armvibration of the multiple-articulated robot.

We claim:
 1. A control apparatus for a multiple-articulated robot inwhich each arm is independently driven by a feedback-controlled jointdriving servomotor, comprising:arithmetic means for computing mutualinterference torque values for respective ones of the arms; statusobserving means for reproducing a status variable from a torque commandand actual velocity of each servomotor, said status observing meansincluding:constant term means for receiving an acceleration value andconverting it to a constant term; integration means, connected to saidconverting means, for receiving and converting the constant term valueto a velocity value; a first adder, connected to the servometer and saidintegration means, for calculating an error having a steady componentand a non-steady component; error elimination means, connected to saidfirst adder, for eliminating the steady error component; and torquemeans, connected to said error elimination means, for providingcorrective torque to the non-steady error component from said errorelimination means; a second adder, connected to said corrective means,for adding the outputs from said torque means; conversion means,connected to said status observing means, for receiving the velocityvalue from said integration means and for converting an output of thestatus observing means into a corrective value of torque produced bydisturbance acting upon the servomotor; and correcting means, connectedto said conversion means and said second adder, for receiving the outputfrom said second adder and the corrective value and for correcting anerror signal of the torque command of the servomotor by the convertedcorrective value and said interference torque value.
 2. A controlapparatus for a multiple-articulated robot according to claim 1, whereinsaid arithmetic means comprises a microcomputer.